Load theory of attention (Lavie, 2005) looks at attention selection by applying a capacity approach to infer the changes brought about by selective attention on visual perception.
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The load theory of attention centres on the influence that a certain quantity of incoming visual stimuli has on attention selection system and the extent of distraction generated as a result (Lavie, 1995). Based on the load theory, one of the key selective mechanisms of attention is that of an involuntary passive-perceptual system which is of limited capacity. It operates selectively when attending to visual stimuli (linguistic and non-linguistic) based on the quantity of available information at any one time. In the presence of information arriving from a large influx of visual stimuli (high perceptual load), capacity to process information would be minimum due to the exhaustion of its reserves following the processing of the attended information, thus resulting in the lack of perception on the part of unattended information. That is, for 5 potential target stimuli and one distractor competing with 5 channels, the chances of perceiving the distractors are minimal as the targets fully consume the available attentional resources. In contrast, for tasks taxing low perceptual load (e.g. 5 channels, for which 1 potential target stimuli and 5 distractors are presented), both attended and unattended information ends up being processed until all the attentional capacity gets used up. Given the spare perceptual capacity to begin with, 4 out of the 5 distractors are more likely to be perceived as sufficient capacity spills over for the uptake of distractor information. Therefore, at a state of low perceptual load, distractor information generally results in interference as attentional selection occurs in the late stages of visual processing, i.e. attention operates non-selectivity during low perceptual load, whilst operation occurs selectively at high load conditions (Lavie, Beck, & Konstantinou, 2014; Lavie & Tsal, 1994). This goes to show the relative importance in being able to retain not only the task information relevant to a particular goal (properties of task-relevant vs. irrelevant stimuli), but also the ability to prioritise at high perceptual load (Lavie, Hirst, De Fockert, & Viding, 2004) both of which are crucial for optimum selective and focused attention.
Earliest studies used measures of distractor interference based on task reaction time to estimate the influence of perceptual load on attention control and perception. By manipulations such as increasing either the number of stimuli presented on a stimulus display (the term referred to as the "set-size") to be processed, or increasing the complexity of perceptual function of a task (whilst keeping the set-size constant), many authors have succeeded in amplifying the perceptual load. Lavie and colleagues carried out several experiments investigating whether perceptual load modulated the distractor interference effect in attention selective visual tasks (Lavie, 1995; Lavie & Cox, 1997; Lavie & de Fockert, 2003; Rees, Frith & Lavie, 2001). In an attempt to inspect the functional capacity of the perceptual load, Lavie (1995) utilized the response competition paradigm to evaluate distractor interference. Perceptual load was controlled by randomly
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altering the stimulus set-size, i.e. a single target letter was presented in one of six stimulus locations with only the target stimulus presented in low load conditions, compared to a target and five non-target letters being presented at high load conditions (figure 10). At low load conditions, reaction times of target detection were reported to be fastest on trials containing compatible items (both target and non-target are x), and slowest on incompatible items (target as x and non-target as z), which suggested facilitation effects caused by compatible distractors whilst incompatible distractors caused interference effects. No significant differences across distractor type was reported for high load conditions, suggesting that the attention system had a poor ability to exclude low-priority items from undergoing perceptual processing given the increased demands for attentional capacity. Moreover, Lavie & Cox (1997) varied the similarity between targets and distractors to induce variations in search load under two task difficulties, easy (low load) and hard (high load). Findings showed significantly slower reaction times in the easy/low perceptual load compared to the hard/high perceptual load search task. This suggests that the reduced distractor interference effect at high perceptual load to be a result of poor prioritization for difficult tasks. Using similar studies of this nature, Lavie demonstrated that an increased perceptual load demanding greater attentional resources led to far more efficient elimination of distractor processing. Several other studies have also found findings consistent with that of Lavie (e.g. Cartwright-Finch & Lavie, 2007; Lavie, 2010; Tsal & Benoni, 2010) despite a few alternative accounts tapping effects of perceptual load with target saliency. For instance, with the target appearing more salient than the distractors, Eltiti, Wallace, & Fox (2005) demonstrated distractor interference at high load, suggesting that factors external to processing capacity may influence distractor interference.
With studies having demonstrated that focusing visual attention tends to result in less processing of distractor items (e.g. Lavie, 1995), a handful of few studies have in fact utilised this idea to extend findings into visual attention orienting deficits in dyslexia. For instance, Facoetti & Molteni (2000) performed a choice reaction time task where a central coloured (green or red) dot was surrounded by a distractor (letter) on both the left and right side based on its compatibility towards the type of response (response-compatible or response-incompatible dot, both of which were denoted by dot’s colour). With each trial initiated by either a 0 or 500 msec SOA central circular dot stimuli, a central cue (either small or large) was presented requiring participants to respond towards the appropriate task compatible colour. The authors reported findings of faster responses during compatible unlike incompatible trials (the term referred to as the flanker effect) that too only in the presence of the smaller cue presented at 500 msec SOA. They concluded that
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constraining visual attention is time dependant, i.e. at shorter durations (at 0 msec SOA) neither cue type had any effects of disparity given the lack of time needed for attention to confine its functions to that cue, whilst with the longer duration (500 msec SOA) attention was able to confine itself to either of the two cues with ease, suggesting the difficulty that dyslexic individuals had when eliminating the distractors when included within a larger cue unlike with the smaller cue. This finding therefore raises a question as to whether a limit such as this is entirely capacity dependant based on how participants are made to focus their attention.
It therefore becomes even more interesting to know the process best describing that in the absence of a cue altogether, with participants still requiring to limit their visual attention. This answer to this question came from Facoetti et al (2003) having demonstrated an impaired visual attention focusing ability in dyslexia. This study was same as that in Facoetti & Molteni (2001), with the exception of the minimum SOA being 100 msec, in addition to a pointed arrow target. Facoetti and colleagues attributed the lack of cueing benefit at 100 msec in line with a sluggish orienting of visual attention arising due to a limitation in attention controlling ability. When it comes to reading, dyslexic individuals are known for their enduring problems with limiting visual attention (i.e. excluding information from peripheral regions while maintaining foveal fixation). Such deficits are even more
Figure 10: Perceptual load and distractor interference effects. Displaying
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pronounced when carried out in dyslexic populations with the added influence of interference based set-size effects, i.e. word length based on the number of letters (e.g. Ghahghaei, Linnell, Fischer, Dubey, & Davis, 2013). Hence, for dyslexic individuals, the task of reading exerts a heavier load on visual attention compared to controls especially when it comes to nonword and pseudo-word reading (both which consists of unfamiliar syllabic structure). The effects of perceptual load are such that, active management of process is necessary when orienting the attention independent of saccades given the constant shifting of the attentional focus. Should the (fixated) target word demand much higher perceptual resources, attention in the absence of saccades should end up being selectively focused on the word, thus delaying the disconnection from the fixated word. This results in the postponement of both attention reorienting and saccade towards the next (target-to-be) fixated word (Rayner, 2009). However, this is completely opposite in skilled readers where scanning of a given set of text happens effortlessly with much greater fluency despite the word length, with the difference in performance reflecting the ability of skilled readers to narrow their attentional focus, thereby aiding them to identify each letter in a given word with ease.
In summary…
With regards to both the early and late selection theories of attention, the former explains that information regarding distractors may end up being excluded from further processing at a very early stage. However, the latter proposes that information regarding both the distractors and the target are subjected to complete processing such that selection of target information takes place at a much later stage. In addition, both theories differed based on how they accounted for distractor interference effects. In early selection, distractor interference effects were accepted as a deficiency with regards to visual attention system in being unable to exclude information concerning distractors. In late selection, the distractor interference effects were accepted as an outcome due to complete processing of information coming from both the target and distractors. That is, the visual attention system is capable of sustaining some degree of distractor interference effects at the expense of an overall slow functioning attention system. Despite the broad range of studies conducted, the disputes encircling the precise or near-precise site of selective visual attention remains pretty vague. The load theory of selective attention mentions a key perceptual process accountable for distractor exclusion. The extent of distractor interference was investigated by means of various experimental manipulations. Perceptual load theory provides a proposal concerning the site of visual selective attention by linking both the early and late selection theories.
The next section focuses on visual attention types and the experimental paradigms used within control populations, their functional importance, along with the mechanisms to combat efficient distractor interference. The focus then leads into the dyslexic population.
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